Polyamide resin has excellent mechanical properties (such as mechanical strength, rigidity and impact resistance), toughness, heat resistance and chemical resistance, and hence are used in various industrial fields including clothing material, industrial materials, automobiles, electric and electronic and other industries. In particular, polyamide resin is excellent in heat aging resistance as compared to other resins. Accordingly, polyamide resins are used as materials for components in the places having very large quantity of heat, such as the interior of engine rooms of automobiles.
Recently, in engine rooms of automobiles, the environmental temperature of the engine rooms become higher than ever due to the high densification of the components and the increase of the engine output. Accordingly, polyamide resin used as materials for components of the automobile engine rooms are demanded to have such a high (long term) heat aging resistance as significantly exceeding the hitherto existing level.
As a technique to improve the heat aging resistance of polyamide resin, there has hitherto been known a technique in which a copper compound (an oxide or a salt of copper) is added to polyamide resin.
Similarly, as a technique to improve the heat aging resistance of polyamide resin, there have also been disclosed a technique in which a cooper compound and iron oxide is mixed in tow types of polyamides different in melting point from each other (for example, see Patent Document 1), a technique in which a fine powder elementary iron is mixed in polyamide (for example, see Patent Document 2), and a technique in which a finely dispersed metal powder is mixed in polyamide (for example, see Patent Document 3).
On the other hand, there has been disclosed a technique in which magnetite is finely dispersed in nano scale in a polyamide resin to obtain a composition excellent in surface exterior appearance (for example, see Patent Document 4).